1. Field of the Invention
The present invention relates to a process for preparing a substrate used for a high-density optical recording medium such as an optical disk or an optical card. It also relates to an apparatus for preparing the substrate.
2. Related Background Art
Substrates for optical recording mediums such as CDs, laser disks, DRAW disks and photomagnetic disks that optically record and reproduce information have been hitherto prepared by injection molding or compression molding. Although the substrates are required to have a high flatness or smoothness, these molding methods tend to cause warpage or inclusion of bubbles, and hence the detection of information by light may be greatly inhibited. Thus, great difficulties are involved in controlling temperature and pressure, assuring accuracy of a mold, or preventing generation of bubbles. An apparatus with a large scale is also required, resulting in a great increase in cost. Moreover, substrates are formed sheet-by-sheet in the compression molding, and hence complicated post-processing is required, resulting in a poor productivity.
Use of a plastic flat sheet enables easy manufacture of a smooth and uniform sheet with less inclusion of bubbles, and hence a method has been proposed in which a stamper is brought into close contact with this plastic flat sheet under application of pressure to transfer grooves to the sheet. However, a very high pressure must be used in order to apply pressure over the whole disc surface of the flat sheet.
As a means for solving this problem, a plastic sheet extruded from an extruder may be passed between a molding roll provided with a stamper in close adhesion and a mirror-finished roll so that grooves formed on the stamper can be thereby formed on the flat sheet under a low pressure, and then center holes and peripheries may be trimmed away. Substrates for optical recording mediums can be thus obtained using an apparatus of a small scale.
Incidentally, as materials for the substrates used for high-density optical recording mediums such as optical disks, polycarbonate resins have been mainly utilized in view of their properties such as impact properties and weathering properties.
Polycarbonates, however, have the disadvantage that they tend to cause double refraction and the double refraction may immediately occur when a strain is present. The occurrence of double refraction causes impairment of a C/N ratio at the time of recording-reproducing, and therefore must be decreased as far as possible.
As a means for solving this problem, the present applicants have proposed, in Japanese Patent Application No. 63-289828 filed Nov. 16, 1988, a process for preparing a substrate for an optical recording medium, comprising transferring a preformat pattern to a resin sheet extruded from a die, by the use of a three-roll system, wherein the temperature and rotational speed of each of three rolls are controlled so that the double refraction in the resin sheet can be decreased.
Employment of this process has made it possible to obtain a substrate for optical recording mediums that causes less double refraction. However, more than just to decrease the double refraction, it is sought to uniformly decrease double refraction over the whole surface of a substrate.